Coating with silicon carbide by immersion reaction



United States Patent 3,455,723 COATING WITH SILICON CARBIDE BY IMMERSION REACTION Edward L. Kern, Midland, Mich, assignor to Dow Corning Corporation, Midland, Micl1., a corporation of Michigan No Drawing. Filed Dec. 2, 1966, Ser. No. 598,631 Int. Cl. B05c 3/02; C23c 9/10 US. Cl. 11747 3 Claims ABSTRACT OF THE DISCLOSURE The process of coating objects with homogeneous sili con carbide. Object is heated to at least 1150 C. and quenched in liquid alkyl chlorosilane such as dimethyldichlorosilane. Thicker coatings obtainable by sequential heating and quenching.

Background of the invention This application relates to the coating of objects with homogeneous layers of silicon carbide and more particularly to the forming of such layers in situ. The use of silicon carbide as a protective coating for various metallic and non-metallic substances is rapidly becoming routine for many applications. Silicon carbide is particularly suited as a protective coating because of its high melting point and extreme chemical inertness. Heretofore, homogeneous silicon carbide coatings have been formed by a vapor decomposition or vapor reaction such as those described in US. Patent No. 3,157,541 and Canadian Patent No. 657,304. A problem with the vapor deposition techniques described in the aforementioned patents is the time required to produce coatings of useable thickness. Since the process is time consuming, it is also inherently expensive. Furthermore, metals tend to lose their temper as they are heated for vapor deposition and do not regain it in further processing. Additionally, the coating tends to be nonuniform because of uneven flow patterns over the surface of the object being coated. It is toward these problems that the present invention is directed.

SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a method of coating objects with silicon carbide, said method consisting in heating the objects to at least 1150 C. and then quenching the object in a liquid bath consisting essentially of one or more members chosen from the group of silanes having the general formula DESCRIPTION OF THE PREFERRED EMBODIMENTS The process of the present invention ma be performed on any substrate which retains its shape upon heating to at least 1150 C. and which is substantially non-reactive with respect to the quenching baths used in carrying out the invention. Such substrate materials in- 3,455,723 Patented July 15, 1969 elude, for example, carbon, various element metals such as molybdenum, tantalum and silicon, as well as alloys such as high temperature steels, among others.

The object to be coated is first heated by any suitable means to a temperature at least 1150 C. and preferably below about 1350 C. While the method can be performed at higher temperatures, the reaction tends to become violent and in some cases the coating formed is irregular. The heating may be carried out, for example, by use of high frequency induction, electrical resistance, radiant heating, or electron beam heating.

After the object has been heated, it is lowered into a liquid bath consisting essentially of one or morernembers chosen from the group of silanes having the general formula wherein each R is a member of the group consisting of chlorine, hydrogen and methyl, ethyl and propyl groups and R is a member of the group consisting of methyl, ethyl and propyl groups. A preferred quenching material is dimethyldichlorosilane, although other materials perform equally as well.

The quenching liquid is preferably held in a cooled container such as, for example, a quartz, or stainless steel crucible placed in a bath consisting of acetone and Dry Ice. The cooling tends to prevent vaporization which with these materials occurs quite rapidly at room temperature. Additionally, it is desirable to utilize a closed system in which an atmosphere of inert gas such as argon is passed over a liquid bath to carry off vapors.

The coating technique obtained by the aforedescribed process varies with the temperature to which the object is heated and also is dependent to some extent upon the quenching liquid used. If an additional thickness of coating is required for a particular application, the coated object may be simply reheated and quenched repeatedly until the desired thickness is reached. The time during which the object is left in the quenching bath has no effect on the coating as long as the object is left in the bath sufficiently long for the object to be cooled to a temperature below about 1000 C. Due to the relatively high rate of cooling caused by the extremely cold quenching liquid and heat of reaction being removed, the maximum amount of coating is generally formed within about 3 minutes. Of course, this amount varies with the size of the object and the other factors heretofore mentioned. The process, however, is considerably more rapid than the vapor deposition processes. Additionally, substantially uniform coatings are provided over the surface of the article and the quenching provides temper to objects which tend to lose their temper during heating. It has been found useful, for example, in coating steel-ball studs for use in automotive steering gear. The following examples are illustrative of the process.

Example 1 A polished graphite block A" by 1%" by 2" was heated by high frequency induction to 1225 C. This was done in a large quartz tube having a quantity of dimethyldichlorosilane at the bottom thereof, the exterior of the tube bottom being placed in a bath of acetone and Dry Ice. A flow of argon was maintained into the tube and the tube was vented. After the block was heated, it was lowered into the silane and allowed to cool to the silane temperature before removal. The block was weighed and showed a weight gain of 0.11010 gram. A coating was visible on the block surface and upon analysis was determined to be silicon carbide.

Exa p e as A steel-ball stud was heated to 1350 C. in the apparatus of Example 1 and was immersed into the silane until red heat was no longer visible. The ball stud was then removed from the silane and upon examination itwas found that the ball had an irregular coating on its surface. Upon analysis the coating was found to be silicon carbide.

Example 3 A ball stud identical to that of Example 2 was coated in accordance with the steps given in Examplesl and 2 except that the temperature of the ball prior to immersion was 1100 C. Upon examination it Was found that there was a uniform coating completely around the ball, and upon analysis it was found that the coating was silicon carbide.

Example 4 A turbine blade of Inconel metal was processed in accordance with the steps set forth in Example 1. A silicon carbide coating was formed over its entire surface.

Example 5 A graphite block similar to that described in Example 1 was coated in accordance with the process set forth in Example 1 and was found to have a weight gain of .02111 gram after immersion for 30 seconds. The block was then heated and immersed sequentially 5 more times and showed an over-all gain of approximately 0.02 gram per immersion. Upon analysis the coating was determined to be silicon carbide.

Example 6 When methyltrichlorosilane, ethylmonochlorosilane, propyltrichlorosilane and trimethylrnonochlorosilane are each substituted for the dimethyldichlorosilane of Examples 1, 3, 4, and 5, similar results are obtainable.

Example 7 When objects of molybdenum, tantalum, niobium and silicon are each used in the place of graphite in the process of Example 1, similar results to those of Example 1 are obtainable.

That which is claimed is:

1. The method of coating an object with silicon carbide which consists of 4 i V t nasaid Obie-r 19. tr sa 115 and, t ir...

quenching said object in a liquid bath consisting essentially of one or more members chosen from the group of silanes having the general formula wherein each R is a memberof the group consisting of chlorine, hydrogen and methyl, ethyl and propyl groups, and R is a member of the group consisting of methyl, ethyl, and propyl groups.

2. The method as defined in claim l wherein the heating and quenching steps are sequentially repeated to achieve a desired coating thickness. l

3. The method of coating an object with silicon carbide which comprises:

heating said object to at least 1150" C. and

immersing said object in a liquid bath consisting es-' sentially of one or more members chosen from the group of silanes having the general formula wherein each R is a member of the group consisting of chlorine, hydrogen and methyl, ethyl and propyl groups, and R is a member of the group consisting of methyl, ethyl, and propyl groups.

References Cited UNITED STATES PATENTS 3,099,534 7/1963 Schweickert et a1 23 '20s 3,157,541 11/1964 Heywang et al. 117106 X 3,406,044 10/1968 Harris 117-106 X ALFRED L. LEAVITT, Primary Examiner THOMAS E. BOKAN, Assistant Examiner Disclaimer 3,455,728.Ldwa1"d L. Kern, Midland, Mich. COATING lVITH SILICON CARBIDE BY IMMERSION REACTION. Patent dated July 15,

1969. Disclaimer filed Oct. 18, 1972, by the assignee, Dow Uorm'ng Corporation.

Hereby enters this disclaimer to claims 1-3 of said patent.

[Ofiioial Gazette February 19, 1974.] 

